I had a 6+ at lunch and loved it, but I returned it within the 14 day returns period and swapped it for a 6.
The reason was that I had a few too many 'bar of soap' moments for my liking.
Ironically, this may have been in part due to the case I had on it (a Spigen Tough Armor) which made it even larger and harder to hold.
I also tried a Slim Armor, but that had a similar effect. Although it's a thinner case, it has quite a shiny back, which can make for more 'bar of soap' moments.
One thing I will say, is that the phone is gorgeous. That screen makes for a fantastic viewing experience, but I was forever having issues holding it.
Maybe a better case would solve the issue, but for me it was a no go (even though I loved the phone!).
HTH.